Segment Information (Income Statement by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Rental income									$ 1,609,876	$ 1,513,807	$ 1,593,598
Income from loans and investments									89,201	124,218	117,608
Interest and other income									10,984	24,892	6,034
Total revenues	$ 996,004	$ 983,155	$ 950,717	$ 942,874	$ 923,263	$ 936,538	$ 942,304	$ 943,705	3,872,750	3,745,810	3,574,149
Less:
Interest and other income									10,984	24,892	6,034
Property-level operating expenses									1,808,208	1,689,880	1,483,072
Office building services costs									2,319	1,418	3,391
Segment NOI									2,051,239	2,029,620	2,081,652
Interest and other income									10,984	24,892	6,034
Interest expense									(451,662)	(442,497)	(448,196)
Depreciation and amortization									1,045,620	919,639	887,948
General, administrative and professional fees									(165,996)	(151,982)	(135,490)
Loss on extinguishment of debt, net									(41,900)	(58,254)	(754)
Merger-related expenses and deal costs									(15,235)	(30,547)	(10,535)
Other											(20,052)
Loss from unconsolidated entities									(2,464)	(55,034)	(3,588)
Gain on real estate dispositions									26,022	46,247	717,273
Income tax benefit									56,310	39,953	59,799
Income from continuing operations									439,297	415,991	1,361,222
Other Nonoperating Income (Expense)									17,609	(66,768)	(20,052)
Discontinued operations					0	0	0	(10)	0	(10)	(110)
Net income	12,893	86,918	211,898	127,588	63,302	103,281	169,300	80,098	439,297	415,981	1,361,112
Net income attributable to noncontrolling interests	1,450	1,659	1,369	1,803	1,029	1,309	2,781	1,395	6,281	6,514	4,642
Net income attributable to common stockholders	$ 11,443	$ 85,259	$ 210,529	$ 125,785	$ 62,273	$ 101,972	$ 166,519	$ 78,703	433,016	409,467	1,356,470
Operating Segments | Triple Net Leased Properties
Revenues
Rental income									780,898	737,796	840,131
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									780,898	740,318	844,711
Less:
Interest and other income									0	0	0
Property-level operating expenses									26,561	0	0
Office building services costs									0	0	0
Segment NOI									754,337	740,318	844,711
Interest and other income									0	0	0
Operating Segments | Senior Living Operations
Revenues
Rental income									0	0	0
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									2,151,533	2,069,477	1,843,232
Less:
Interest and other income									0	0	0
Property-level operating expenses									1,521,398	1,446,201	1,250,065
Office building services costs									0	0	0
Segment NOI									630,135	623,276	593,167
Interest and other income									0	0	0
Operating Segments | Office Operations
Revenues
Rental income									828,978	776,011	753,467
Income from loans and investments									0	0	0
Interest and other income									0	0	0
Total revenues									836,725	783,603	760,964
Less:
Interest and other income									0	0	0
Property-level operating expenses									260,249	243,679	233,007
Office building services costs									2,319	1,418	3,391
Segment NOI									574,157	538,506	524,566
Interest and other income									0	0	0
All Other
Revenues
Rental income									0	0	0
Income from loans and investments									89,201	124,218	117,608
Interest and other income									10,984	24,892	6,034
Total revenues									103,594	152,412	125,242
Less:
Interest and other income									10,984	24,892	6,034
Property-level operating expenses									0	0	0
Office building services costs									0	0	0
Segment NOI									92,610	127,520	119,208
Interest and other income									10,984	24,892	6,034
Resident Fees and Services
Revenues
Revenue from contracts with customers									2,151,533	2,069,477	1,843,232
Resident Fees and Services | Operating Segments | Triple Net Leased Properties
Revenues
Revenue from contracts with customers									0	0	0
Resident Fees and Services | Operating Segments | Senior Living Operations
Revenues
Revenue from contracts with customers									2,151,533	2,069,477	1,843,232
Resident Fees and Services | Operating Segments | Office Operations
Revenues
Revenue from contracts with customers									0	0	0
Resident Fees and Services | All Other
Revenues
Revenue from contracts with customers									0	0	0
Office Building and Other Services Revenue
Revenues
Revenue from contracts with customers									11,156	13,416	13,677
Office Building and Other Services Revenue | Operating Segments | Triple Net Leased Properties
Revenues
Revenue from contracts with customers									0	2,522	4,580
Office Building and Other Services Revenue | Operating Segments | Senior Living Operations
Revenues
Revenue from contracts with customers									0	0	0
Office Building and Other Services Revenue | Operating Segments | Office Operations
Revenues
Revenue from contracts with customers									7,747	7,592	7,497
Office Building and Other Services Revenue | All Other
Revenues
Revenue from contracts with customers									$ 3,409	$ 3,302	$ 1,600